Etymologies

Middle English, from Old English nunne and from Old French nonne, both from Late Latin nonna, feminine of nonnus, tutor, monk.

Mishnaic Hebrew nûn, of Phoenician origin; see nwn in Semitic roots.

(American Heritage® Dictionary of the English Language, Fourth Edition)

From Late Latin nonna ("nun, tutor"), originally (along with masculine form nonnus ("man")) a term of address for elderly persons, perhaps from children's speech, reminiscent of nana, like papa etc. (Wiktionary)

Ultimately from Proto-Semitic *nūn- (“fish”). (Wiktionary)

Examples

JACKI SCHECHNER, CNN INTERNET REPORTER: Tom, take a close look at what they call the nun bun, or the immaculate confection.

And the average age of an American nun is around 70 — except in traditional orders such as the Dominican Sisters of Mary, Mother of the Eucharist in Ann Arbor, Michigan, which now [in 2006] have 73 members with an average age of 24.

She turns out to be a nun from the Church of England, and her mind has been implanted with the Index-Librorum-Prohibitorum – all the magical texts the Church has removed from circulation. (from: Wikipedia)

I didn't speak to a single nun from the day I graduated from high school, in 1967, until the late 1980s, when I encountered nuns as political allies, signers of an advertisement taken out in The New York Times saying that not all Catholics were anti-choice.